1. Field of the Invention
The present invention relates to a magnetic recording medium. More specifically, it relates to a magnetic recording medium of a coating type comprising an undercoat layer provided between a non-magnetic support and a magnetic layer.
2. Description of the Background Art
A magnetic recording medium has recently been required to have a higher recording density and hence is under investigations with a view to improving the magnetic layer thereof by an adequate method such as incorporation there into of a finer magnetic material at a higher filling ratio or formation of a thinner film as the magnetic layer for the purpose of heightening the recording density thereof. When the magnetic layer is improved by such a method, however, the adhesion of the magnetic layer to a support and the coating film strength of the magnetic layer tend to deteriorate, resulting in poor durability of the magnetic recording medium. In order to improve the durability, there has heretofore been employed a method in which a support is coated with an undercoating liquid containing a polyester or a polyurethane resin dissolved in an organic solvent.
However, the use of any such conventional undercoating liquid involves the possibility that an undercoat layer, formed by application and drying the liquid, is swollen with or dissolved in an organic solvent contained in a magnetic coating upon application of the magnetic coating on the undercoat layer to the detriment of the smoothness of the interface between the magnetic layer and the undercoat layer to deteriorate the smoothness of the surface of the magnetic layer. This presents problems including a decrease in output and an increase in output variation. In view of his, there is employed a method in which a polyisocyanate compound is added to an undercoating liquid to crosslink therewith a resin contained in the undercoating liquid for suppressing the swelling or dissolution of an undercoat layer formed therefrom with a solvent. However, it can hardly be said that the effect of this method is sufficient.